"Armies all over the world have committed mistakes and it is possible that an isolated bombing took place. If this is so, we will punish the offenders," he said.

Darfur is one of the issues being discussed at the two-day AU summit, along with reducing poverty, UN reform and the conflicts in Democratic Republic of Congo and Ivory Coast.

The heads of state have decided to retain Nigeria's President Olusegun Obasanjo as leader of the AU for another year, the AFP news agency reports. They also decided to hold their next half-yearly summit in July in Libya while the January 2006 meeting will be staged in Sudan, to coincide with the 50th anniversary of its independence.

'Disarm militia'

Mr Annan said that "action will have to be taken," to end the Darfur conflict.

"The [UN Security] Council had considered sanctions and had not been able to move forward because of some divisions in the Council. But I believe that sanctions should still be on the table," he said.

Africa remains blighted by conflict and poverty

The United States has pushed for sanctions to be imposed on Sudan if the violence does not end but this has been opposed by China and Russia, which have economic ties to the Sudan government.

The Darfur rebels have appealed for a peacekeeping force strong enough to disarm the Arab militias, accused of systematic killings and mass rape.

Mr Annan also warned that Africa was "not on track" to meet development goals set five years ago.

Then, members of the UN pledged to halve the number of people living in poverty, reverse the spread of Aids and provide basic education by 2015.

"It lags behind other parts of the developing world ... It continues to suffer from the tragic consequences of deadly conflict and poor governance," Mr Annan said.

UN reform

Enhancing African standing at the UN is also expected to be discussed at the summit.

Nigeria is at the forefront of African states vying for a permanent seat on an expanded UN Security Council.

Draft resolutions and topics for debate have been hammered out by officials and government ministers over the past few days, but the final agenda will be decided by the delegates themselves.

The 53-member AU was set up in 2002 to help promote unity and peace and encourage prosperity on the African continent.